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| United States Patent |
5,617,207
|
|
Glass
, et al.
|
April 1, 1997
|
Appartatus and method for measuring a change in an energy path length
Abstract
Apparatus and method for measuring a change in an energy path length are
disclosed. One broad embodiment disclosed is an apparatus for measuring a
change in an energy path length comprising an energy source (301) having
means (302, 307) for emanating a first energy beam, substantially
uncollimated, wherein at least a portion of the first energy beam is
substantially coherent and having means (327) for coherently guiding a
second energy beam to an energy interferometer, a coherent energy director
(310), an energy collector (317), wherein the energy director (310) is
operative associated with the means for emanating and the collector (317)
thereby coherently directing at least a portion of the first energy beam
from the means (302, 307) for emanating to the collector (317), wherein
the collector (317) is operatively associated with the interferometer
thereby coherently directing at least a portion of the collected first
energy beam to the interferometer whereby the collected first energy beam
interferes with the second energy beam thereby producing an output signal,
means (333) for changing the energy path length of the first energy beam
between the means (302, 307) for emanating and the director (310), the
means (333) for changing being operatively associated with the first
energy beam emanator, and a calculator (319) operatively associated with
the interferometer to determine the change in the energy path length from
a change in the output signal.
| Inventors:
|
Glass; Monty (Dulwich Hill, AU);
Dabbs; Timothy P. (West Ryde, AU)
|
| Assignee:
|
Commonwealth Scientific and Industrial Research Organisation (Australian Capital Territory, AU)
|
| Appl. No.:
|
938165 |
| Filed:
|
December 4, 1992 |
| PCT Filed:
|
April 23, 1991
|
| PCT NO:
|
PCT/AU91/00154
|
| 371 Date:
|
December 4, 1992
|
| 102(e) Date:
|
December 4, 1992
|
| PCT PUB.NO.:
|
WO91/16597 |
| PCT PUB. Date:
|
October 31, 1991 |
Foreign Application Priority Data
| Apr 23, 1990[AU] | PJ9777 |
| Feb 21, 1991[AU] | PK4716 |
| Current U.S. Class: |
356/477; 250/227.27; 356/511 |
| Intern'l Class: |
G01B 009/02 |
| Field of Search: |
356/345,349,358,359,360
250/227.19,227.27
385/12,14
|
References Cited
U.S. Patent Documents
| 3647275 | Mar., 1972 | Ward | 350/3.
|
| 4313185 | Jan., 1982 | Chovan | 367/149.
|
| 4759627 | Jul., 1988 | Thylen et al. | 356/345.
|
| 4941744 | Jul., 1990 | Yokokura | 356/358.
|
| 5106191 | Apr., 1992 | Ohtsuka | 356/358.
|
| 5161053 | Nov., 1992 | Dabbs | 359/384.
|
| 5359415 | Oct., 1994 | Tabarelli | 356/358.
|
| Foreign Patent Documents |
| 0058801 | Sep., 1982 | EP.
| |
| 3623265 | Jan., 1988 | DE.
| |
| 3709253 | Sep., 1988 | DE.
| |
| 2173592 | Oct., 1986 | GB.
| |
| 90/11484 | Oct., 1990 | WO.
| |
Primary Examiner: Epps; Georgia Y.
Assistant Examiner: Kim; Robert
Attorney, Agent or Firm: Foley & Lardner
Claims
We claim:
1. An apparatus for measuring a change in an energy path length comprising:
(a) an energy source having,
(i) means for emanating a first energy beam, unguided and substantially
uncollimated, wherein at least a portion of the first energy beam is
substantially coherent, and
(ii) means for coherently guiding a second energy beam to an energy
interferometer;
(b) a coherent energy director;
(c) an energy collector;
wherein the coherent energy director is operatively associated with both
the means for emanating and the energy collector, thereby coherently
directing at least a portion of the first energy beam from the means for
emanating to the energy collector;
wherein the energy collector is operatively associated with the
interferometer thereby coherently directing at least a portion of the
first energy beam collected by the energy collector to the interferometer
whereby the collected first energy beam interferes with the second energy
beam thereby producing an output signal;
(d) means for changing an energy path length of the unguided and
substantially uncollimated first energy beam between the means for
emanating and the coherent energy director, thereby changing a phase of
the unguided and substantially uncollimated first energy beam at the
coherent energy director, the means for changing being operatively
associated with the means for emanating; and
(e) a calculator operatively associated with the interferometer to
determine the change in the energy path length between the means for
emanating and the coherent energy director from a change in phase of the
output signal.
2. An apparatus for measuring a change in an energy path length comprising:
(a) an energy source having,
(i) means for emanating an unguided first energy beam wherein at least a
portion of the first energy beam is substantially coherent, and
(ii) means for coherently guiding a second energy beam to an energy
interferometer;
(b) a coherent energy director;
(c) an energy collector;
wherein the coherent energy director is operatively associated with both
the means for emanating and the energy collector, thereby coherently
directing, as a substantially uncollimated beam, at least a portion of the
first energy beam from the means for emanating to the energy collector;
wherein the energy collector is operatively associated with the
interferometer thereby coherently directing at least a portion of the
first energy beam collected by the energy collector to the interferometer
whereby the collected first energy beam interferes with the second energy
beam thereby producing an output signal;
(d) means for changing an energy path length of the unguided and
substantially uncollimated first energy beam between the coherent energy
director and the energy collector, thereby changing a phase of the
unguided and substantially uncollimated first energy beam at the energy
collector, the means for changing being operatively associated with the
energy collector; and
(e) a calculator operatively associated with the interferometer to
determine the change in the energy path length between the coherent energy
director and the energy collector from a change in phase of the output
signal.
3. An apparatus for measuring a change in an energy path length comprising:
(a) an energy source having,
(i) means for emanating a first energy beam, unguided and substantially
uncollimated, wherein at least a portion of the first energy beam is
substantially coherent, and
(ii) means for coherently guiding a second energy beam to an energy
interferometer;
(b) a coherent energy director;
(c) an energy collector;
wherein the coherent energy director is operatively associated with both
the means for emanating and the energy collector, thereby coherently
directing, as a substantially uncollimated beam, at least a portion of the
first energy beam from the means for emanating to the energy collector;
wherein the energy collector is operatively associated with the
interferometer thereby coherently directing at least a portion of the
first energy beam collected by the energy collector to the interferometer
whereby the collected first energy beam interferes with the second energy
beam thereby producing an output signal;
(d) means for changing an energy path length of the unguided and
substantially uncollimated first energy beam between the means for
emanating and the coherent energy director, thereby changing a phase of
the unguided and substantially uncollimated first energy beam at the
coherent energy director, the means for changing being operatively
associated with the means for emanating;
(e) means for changing the energy path length of the unguided and
substantially uncollimated first energy beam between the coherent energy
director and the energy collector, thereby changing a phase of the
unguided and substantially uncollimated first energy beam at the energy
collector, the means for changing being operatively associated with the
energy collector; and
(f) a calculator operatively associated with the interferometer to
determine the change in the energy path length between the means for
emanating and the coherent energy director and between the coherent energy
director and the energy collector from a change in phase of the output
signal.
4. The apparatus as defined in claim 1, 2 or 3 wherein the energy source is
a solid particle beam, acoustic waves, or electromagnetic radiation.
5. The apparatus as defined in claim 1, 2 or 3 wherein the energy source is
a source of electromagnetic radiation with a wavelength in the range of
and including far UV to far IR.
6. The apparatus as defined in claim 1, 2 or 3 wherein the means for
emanating is selected from the group consisting of an exit window of an
energy source, a laser, a laser diode and a pinhole aperture in
combination with a focussing element.
7. The apparatus as defined in claim 1, 2 or 3 wherein the energy source is
operatively associated with an energy guide and the means for emanating is
an energy exit portion of the guide.
8. The apparatus as defined in claim 1, 2 or 3 wherein the means for
coherently guiding the second energy beam is an energy guide or a
focussing system.
9. The apparatus as defined in claim 1, 2 or 3 wherein the coherent energy
director is an energy condenser or focusser.
10. The apparatus as defined in claim 1, 2 or 3 wherein the energy
collector is an aperture or an energy entrance portion of an energy guide.
11. The apparatus as defined in claim 1, 2 or 3 wherein the means for
coherently guiding is an energy guide.
12. The apparatus as defined in claim 1, 2 or 3 wherein the means for
coherently guiding is a coherent optical fibre bundle.
13. The apparatus as defined in claim 1, 2 or 3 wherein the means for
coherently guiding is a flexible, multi mode optical fibre.
14. The apparatus as defined in claim 1, 2 or 3 wherein the means for
coherently guiding is a flexible, single mode optical fibre.
15. The apparatus as defined in claim 14 wherein the numerical aperture,
NA, the fibre core radius, a, and the wave length of the energy, .lambda.,
obey the relationship:
2.times..pi..times.NA.times.a/.lambda..ltoreq.2,405.
16. The apparatus as defined in claim 1, 2 or 3 wherein the energy
interferometer is an energy splitter or the detecting element of a
detector.
17. The apparatus as defined in claim 1, 2 or 3 wherein the means for
changing the energy path between the means for emanating and the director,
between the director and the collector or between the means for emanating
and the director and between the director and the collector, is a scanner.
18. The apparatus as defined in claim 1, 2 or 3 wherein the energy source
is a source of electromagnetic radiation with a wavelength in the range of
and including far UV to far IR and the means for changing the energy path
between the means for emanating and the director, between the director and
the collector or between the means for emanating and the director and
between the director and the collector, is a substance that changes the
refractive index of the energy path of the first energy beam.
19. An apparatus for measuring a parameter using an interferometer cell,
the apparatus comprising:
a first coherence maintaining energy guide which, in operation, has energy
emerging in a substantially uncollimated beam coherently from its exit
end;
an energy focusser, the first energy guide being operatively associated
with the energy focusser so that at least a portion of the substantially
uncollimated energy beam emerging from the exit end of the first energy
guide falls coherently on the energy focusser; and
a second coherence maintaining energy guide having an energy entrance end,
the second energy guide being operatively associated with the energy
focusser so that at least a portion of the substantially uncollimated
energy beam falling on the energy focusser is focussed coherently onto the
entrance end of the second energy guide,
wherein one of the first energy guide exit end and the second energy guide
entrance end is moved in response to the parameter being measured
resulting in a change in the energy path between one of the energy exit
end and the focusser, and the focusser and the energy entrance end,
respectively, to introduce a change in phase of the substantially
uncollimated energy beam such that the parameter being measured is
determined based on the change in phase of the substantially uncollimated
energy beam.
20. The apparatus as defined in claim 19 wherein the energy guide is an
energy fibre.
21. The apparatus as defined in claim 19 wherein the first and second
coherence maintaining energy guides are the same energy guide.
22. The apparatus as defined in claim 21 wherein the focusser includes a
reflector.
23. A method for measuring a change in an energy path length comprising the
steps of:
coherently directing, with a coherent energy director, at least a portion
of a first energy beam to an energy collector from means for emanating the
first energy beam from an energy source, wherein at least a portion of the
first energy beam is substantially coherent, the first energy beam from
the means for emanating being unguided and substantially uncollimated;
collecting at least a portion of the first energy beam with the energy
collector;
coherently guiding a second energy beam from the energy source to an energy
interferometer;
coherently directing at least a portion of the collected first energy beam
to the interferometer whereby the collected first energy beam interferes
with the second energy beam thereby producing an output signal;
changing an energy path length of the unguided and substantially
uncollimated first energy beam between the means for emanating and the
coherent energy director such that a change in the first energy beam at
the coherent energy director is changed to produce a change in phase of
the output signal; and
determining the change in the energy path length between the means for
emanating and the coherent energy director from the change in the phase of
the output signal.
24. A method for measuring a change in an energy path length comprising the
steps of:
coherently directing, with a coherent energy director, at least a portion
of a first energy beam to an energy collector from means for emanating the
first energy beam from an energy source, wherein at least a portion of the
first energy beam is substantially coherent, the first energy beam from
the coherent energy director to the energy collector being unguided and
substantially uncollimated;
collecting at least a portion of the first energy beam with the energy
collector;
coherently guiding a second energy beam from the energy source to an energy
interferometer;
coherently directing at least a portion of the collected first energy beam
to the interferometer whereby the collected first energy beam interferes
with the second energy beam thereby producing an output signal;
changing an energy path length of the unguided and substantially
uncollimated first energy beam between the coherent energy director and
the energy collector such that a change in the first energy beam at the
energy collector is changed to produce a change in phase of the output
signal changes; and
determining the change in the energy path length from the change in the
phase of the output signal.
25. A method for measuring a change in an energy path length comprising the
steps of:
coherently directing, with a coherent energy director, at least a portion
of a first energy beam to an energy collector from means for emanating the
first energy beam from an energy source, wherein at least a portion of the
first energy beam is substantially coherent, the first energy beam from
the coherent energy director to the energy collector being unguided and
substantially uncollimated;
collecting at least a portion of the first energy beam with the energy
collector;
coherently guiding a second energy beam from the energy source to an energy
interferometer;
coherently directing at least a portion of the collected first energy beam
to the interferometer whereby the collected first energy beam interferes
with the second energy beam thereby producing an output signal;
changing an energy path length of the unguided and substantially
uncollimated first energy beam between the means for emanating and the
energy collector such that a change in the first energy beam at the energy
collector is changed to produce a change in phase of the output signal
changes; and
determining the change in the energy path length from the change in the
phase of the output signal.
26. A method for determining a parameter of an optical system which
correlates to a change in an energy path length, the method comprising the
steps of:
emanating a first energy beam from an energy source, the first energy beam
being unguided and substantially uncollimated;
coherently directing the first energy beam from a point of emanation in the
emanating step to a collection point;
collecting at least a portion of the first energy beam at the collection
point as a collected first energy beam;
interfering a coherently guided second energy beam from the energy source
with at least a portion of the collected first energy beam;
measuring the interference between the second energy beam and the portion
of the collected first energy beam;
changing one or both of the point of emanation and the collection point to
produce a change in an energy path length of the unguided and
substantially uncollimated first energy beam thereby changing a phase of
the first energy beam at the collection point;
determining the change in the energy path length from a change in the
interference measured in the measuring step due to the change of phase of
the substantially uncollimated first energy beam at the collection point;
and
calculating the parameter of the optical system based on a correlation
between the parameter and the change in energy path length of the
substantially uncollimated first energy beam.
Description
TECHNICAL FIELD
This invention relates to apparatuses and methods for measuring a change in
an energy path length.
BACKGROUND ART
The Mach-Zehnder interferometer is one of the most popular configurations
for high resolution fibre optic sensors (FIG. 1). In this type of optical
fibre sensor, light from coherent source 100 is injected into single mode
fibre 101. This light is directed into two fibres by coupler 102; the
reference fibre 103 and the signal fibre 104. The light from these two
fibres is recombined by coupler 105 where optical interference takes place
and is monitored by detectors 106 and 107. The signals at detectors are
changed if the optical path length in one arm (the signal arm 104) of the
interferometer changes with respect to the other (the reference arm 105).
There are two basic ways of changing the path length; intrinsic or
extrinsic. For an intrinsic fibre optic interferometer the signal fibre
itself is stretched for example by piezoelectric cylinder 108 (or heated
etc) to change the path length, so the light never has to leave the fibre.
For the extrinsic fibre optic interferometer, light leaves the fibre, is
collimated, passes through a measurement cell where optical path length
changes and is then focussed back into the fibre. Typically, Mach-Zehnder
interferometer sensors measure temperature, pressure, sound, acceleration,
limited displacement, chemical species concentration etc.
It is usual to use a Michelson interferometer (FIG. 2) to measure
displacements, particularly if the displacement is relatively large.
Coherent light from laser 200 passes along single mode fibre 201 to
coupler 202. Light travels along signal fibre 203, leaves the fibre at end
204 and is collimated by lens 205. This light subsequently is reflected by
flat reflector 206 back to lens 205 and thence back into the core of fibre
203 at fibre end 204. This back reflected light interferes in coupler 202
with the reference light reflected from mirrored end 207 of fibre 208. The
intensity resulting from the interference of these two beams in coupler
202 passes along fibre 209 and is detected by detector 210. In prior
interferometers, the fibre-lens combination is moved as a unit relative to
reflector 206 for example by piezoelectric stack 211 which consequently
changes the optical path length in the signal arm.
OBJECTS OF INVENTION
Objects of this invention are to provide apparatuses and methods for
measuring a change in an energy path length.
DISCLOSURE OF INVENTION
For a discussion of "interfere" and "interferes" in accordance with the
intended meaning in this specification reference is made to Principles of
Optics, Max Born and M. L. Wolf, Pergamon Press, 6th Corrected edition,
reprinted 1984 Chapters VII and X, the contents of which are incorporated
herein by cross reference.
According to a first embodiment of this invention there is provided an
apparatus for measuring a change in an energy path length comprising:
an energy source having means for emanating a first energy beam,
substantially uncollimated, wherein at least a portion of the first energy
beam is substantially coherent and having means for coherently guiding a
second energy beam to an energy interferometer;
a coherent energy director;
an energy collector;
wherein the energy director is operatively associated with the means for
emanating and the collector thereby coherently directing at least a
portion of the first energy beam from the means for emanating to the
collector;
wherein the collector is operatively associated with the interferometer
thereby coherently directing at least a portion of the collected first
energy beam to the interferometer whereby the collected first energy beam
interferes with the second energy beam thereby producing an output signal;
means for changing the energy path length of the first energy beam between
the means for emanating and the director, the means for changing being
operatively associated with the first energy beam emanator; and
a calculator operatively associated with the interferometer to determine
the change in the energy path length from a change in the output signal.
According to a second embodiment of this invention there is provided an
apparatus for measuring a change in an energy path length comprising:
an energy source having means for emanating a first energy beam wherein at
least a portion of the first energy beam is substantially coherent and
having means for coherently guiding a second energy beam to an energy
interferometer;
a coherent energy director;
an energy collector;
wherein the energy director is operatively associated with the means for
emanating and the collector thereby coherently directing, as a
substantially uncollimated beam, at least a portion of the first energy
beam from the means for emanating to the collector;
wherein the collector is operatively associated with the interferometer
thereby coherently directing at least a portion of the collected first
energy beam to the interferometer whereby the collected first energy beam
interferes with the second energy beam thereby producing an output signal;
means for changing the energy path length of the first energy beam between
the director and the collector the means for changing being operatively
associated with the collector; and
a calculator operatively associated with the interferometer to determine
the change in the energy path length from a change in the output signal.
According to a third embodiment of this invention there is provided an
apparatus for measuring a change in an energy path length comprising:
an energy source having means for emanating a first energy beam,
substantially uncollimated, wherein at least a portion of the first energy
beam is substantially coherent and having means for coherently guiding a
second energy beam to an energy interferometer;
a coherent energy director;
an energy collector;
wherein the energy director is operatively associated with the means for
emanating and the collector thereby coherently directing, as a
substantially uncollimated beam, at least a portion of the first energy
beam from the means for emanating to the collector;
wherein the collector is operatively associated with the interferometer
thereby coherently directing at least a portion of the collected first
energy beam to the interferometer whereby the collected first energy beam
interferes with the second energy beam thereby producing an output signal;
means for changing the energy path length of the first energy beam between
the means for emanating and the director, the means for changing being
operatively associated with the first energy beam emanator;
means for changing the energy path length of the first energy beam between
the director and the collector the means for changing being operatively
associated with the collector; and
a calculator operatively associated with the interferometer to determine
the change in the energy path length from a change in the output signal.
The energy source can provide a solid particle beam, such as a neutron,
proton or electron beam or a beam of alpha particles, acoustic waves, such
as sound waves, or electromagnetic radiation, such as gamma rays, x-rays,
UV light, visible light, infrared light or microwaves. Generally the
energy source is a source of electromagnetic radiation with a wavelength
in the range of and including far UV to far IR and the energy guide is an
optical fibre.
Examples of light sources include incandescent sources, such as tungsten
filament source, vapour lamps such as halogen lamps including sodium and
iodine vapour lamps, discharge lamps such as xenon arc lamp and a Hg arc
lamp, solid state light sources such as photo diodes, super radiant
diodes, light emitting diodes, laser diodes, electroluminiscent light
sources, laser light sources including rare gas lasers such as an argon
laser, argon/krypton laser, neon laser, helium neon laser, xenon laser and
krypton laser, carbon monoxide and carbon dioxide lasers, metal ion lasers
such as cadmium, zinc, mercury or selenium ion lasers, lead salt lasers,
metal vapour lasers such as copper and gold vapour lasers, nitrogen
lasers, ruby lasers, iodine lasers, neodymium glass and neodymium YAG
lasers, dye lasers such as a dye laser employing rhodamine 640, Kiton Red
620 or rhodamine 590 dye, and a doped fibre laser.
The means for emanating may be the exit window of an energy source, a laser
or laser diode or a pinhole aperture in combination with a focussing
element. Alternatively, the energy source may be operatively associated
with an energy guide wherein the means for emanating is an energy exit
portion of the guide such as an aperture or bend.
The means for coherently guiding the second energy beam may be an energy
guide or a focussing system.
The coherent energy director may be an energy condenser or focusser
including a virtual focusser.
The focussing system or focusser can be refractive lenses, including
microscope objectives, reflective lenses, and/or holographic optical
elements. If the energy is of a frequency other than in the range of UV to
near infrared light or other types of energies, analogous focussing
elements are used in place of the optical focussing elements.
The energy collector may be an aperture or the energy entrance portion of
an energy guide operatively associated with the interferometer to
coherently guide collected energy to the interferometer for example.
The energy guide can be flexible and can be an energy fibre.
The energy guide can be a flexible, multi mode optical fibre.
The energy guide can be a flexible, single mode optical fibre. For example,
a five micron core fibre which is single mode at a wave length of 633
nanometers given an appropriate refractive index profile. A step index
optical fibre becomes single mode when the numerical aperture, NA, the
fibre core radius, a, and the wave length of light, .lambda., obey the
relationship:
2.times..pi..times.NA.times.a/.lambda..ltoreq.2.405.
The energy guide may be a coherent fibre bundle.
The energy interferometer may be an energy splitter or the detecting
element of a detector, for example.
The energy splitter may be an energy guide coupler such as an optical fibre
coupler or a bulk optic splitter. The optical fibre coupler may be a fused
biconical taper coupler, a polished block coupler, a bottled and etched
coupler or a bulk optics type coupler with fibre entrance and exit
pigtails, a planar waveguide device based on photolithographic or
ion-diffusion fabrication techniques or other like coupler.
The means for changing the energy path between the means for emanating and
the director or changing the energy path between the director and the
collector or both may be a scanner or a substance that changes the
refractive index in the path of the illuminating light for example.
The scanner can be a piezoelectric stack, a magnetic core/magnetic coil
combination, a mechanical vibrator, an electromechanical vibrator, a
mechanical or electromechanical scanning mechanism such as a servomotor,
an acoustic coupler electrooptic scanning means or any other suitable
means.
Typically a parameter measuring interferometer cell comprises:
a first coherence maintaining energy guide which, in operation, has energy
emerging coherently from its exit end;
an energy focusser;
the first energy guide being operatively associated with the energy
focusser so that at least a portion of the energy emerging from the exit
portion of the first energy guide is collected coherently by the energy
focusser;
a second coherence maintaining energy guide comprising an energy entrance
end; the second energy guide being operatively associated with the energy
focusser so that at least a portion of the energy collected by the energy
focusser is focussed coherently into the core of the second energy guide;
the first energy guide exit end or the second energy guide entrance end
being translated by the parameter to be measured resulting in a change in
the energy path between the energy exit end and the focusser or the
focusser and the energy entrance end respectively.
Note that the first and second coherence maintaining energy guides may be
the same energy guide in which case the focusser would typically include a
reflector.
Energy emerging coherently from the exit end of the first energy guide may
be directed towards the focusser as a result of reflection, refraction,
diffraction, scattering, for example.
The calculator may include optical electrical optoelectronic mechanical or
magnetic elements, for example, or may include such techniques as optical
and electrically heterodyning, quadrature operation multi area detectors
or phase lock loop techniques, for example.
According to a fourth embodiment of this invention there is provided a
method for measuring a change in an energy path length comprising:
coherently directing, with an energy director, at least a portion of a
first energy beam, wherein at least a portion of the first energy beam is
substantially coherent, from means for emanating the first energy beam
from an energy source, the first energy beam from the means for emanating
being substantially uncollimated, to an energy collector and coherently
guiding a second energy beam from the energy source to an energy
interferometer;
coherently directing at least a portion of the collected first energy beam
to the interferometer whereby the collected first energy beam interferes
with the second energy beam thereby producing an output signal;
changing the energy path length of the first energy beam between the means
for emanating and the director whereby the output signal changes; and
determining the change in the energy path length from the change in the
output signal.
According to a fifth embodiment of this invention there is provided a
method for measuring a change in an energy path length comprising:
coherently directing, with an energy director, at least a portion of a
first energy beam, wherein at least a portion of the first energy beam is
substantially coherent, from means for emanating the first energy beam
from an energy source to an energy collector, the first energy beam from
the director to the collector being substantially uncollimated, and
coherently guiding a second energy beam from the energy source to an
energy interferometer;
coherently directing at least a portion of the collected first energy beam
to the interferometer whereby the collected first energy beam interferes
with the second energy beam thereby producing an output signal;
changing the energy path length of the first energy beam between the
director and the collector whereby the output signal changes; and
determining the change in the energy path length from the change in the
output signal.
According to a sixth embodiment of this invention there is provided a
method for measuring a change in an energy path length comprising:
coherently directing, with an energy director, at least a portion of a
first energy beam, wherein at least a portion of the first energy beam is
substantially coherent, from means for emanating the first energy beam
from an energy source, the first energy beam from the means for emanating
being substantially uncollimated, to an energy collector, the first energy
beam from the director to the collector being substantially uncollimated,
and coherently guiding a second energy beam from the energy source to an
energy interferometer;
coherently directing at least a portion of the collected first energy beam
to the interferometer whereby the collected first energy beam interferes
with the second energy beam thereby producing an output signal;
changing the energy path length of the first energy beam between the means
for emanating and the director and between the director and the collector
whereby the output signal changes; and
determining the change in the energy path length from the change in the
output signal.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic diagram of a typical Mach-Zehnder interferometer;
FIG. 2 is a schematic diagram of a typical Michelson interferometer;
FIG. 3 is a schematic diagram of a scanning spot microscope according to
the invention;
FIG. 4 is a schematic diagram of a moving fibre system according to the
invention;
FIG. 5 is a schematic diagram of a refractometer according to the
invention; and
FIG. 6 is a schematic diagram of a detection system which can be used in
the microscope of FIG. 3.
FIG. 7 is a schematic diagram of a white light interferometer analyser.
BEST MODE AND OTHER MODES FOR CARRYING OUT THE INVENTION
In a scanning spot microscope 300 illustrated in FIG. 3 laser diode 301
with integral single mode optical fibre pigtail 302 is fused to port 303
of directional single mode fibre coupler 304. Light from integral fibre
optic pigtail 302 is split between ports 305 and 306. Substantially
coherent and substantially uncollimated light emanating from single mode
fibre exit end 308 of fibre 307 is collected by low numerical aperture
lens 309 (typically 0.01 to 0.1) a portion passes through beam splitter
310 and is focussed by high numerical aperture lens 311 (typically 0.1 to
1.45) into a spot 312 which intersects object 313. The outgoing light
resulting from interaction between the illuminating spot 312 and object
313 is collected by lens 311, passes through beam splitter 310 and is
focussed by lens 309 back into fibre end 308. Outgoing light collected by
the core of fibre 307 is split between ports 303 and 314 of single mode
directional coupler 304. Outgoing light from port 314 passes through
integral fibre optic pigtail 315 to detector 316. A portion of the light
emanating from fibre end 308 is split by beam splitter 310 and focussed by
high numerical aperture lens 317 into the core of single mode fibre 318.
Single mode fibre 318 is fused to port 319 of directional single mode
fibre coupler 320. A portion of this signal light is split between fibre
optic arms 321 and 322 of coupler 320 via ports 323 and 324 respectively.
Signal light from fibre optic arms 321 and 322 strikes detectors 325 and
326. Illuminating light from laser diode 301 emerging from port 306 of
coupler 304 passes down reference fibre 327 to port 328 of coupler 320. A
portion of this reference light is split between fibre optic arms 321 and
322 of coupler 320 via ports 323 and 324 respectively. The output signals
from detectors 325 and 326 passes into calculator 329 the output of
calculator 329 passes into three dimensional imager 330 via line 332. The
signal from detector 316 is fed into imager 330 via line 331.
Piezoelectric stack 333 moves fibre exit end 308 to and fro along its axis
to scan spot 312 in and about object 313 in the z direction. Mechanical
stage 334 scans the object 313 in the x and y directions. Imager 330
communicates with mechanical stage 334 via line 335.
In use coherent illuminating light from laser diode 301 passes to exit end
308 via fibre 302 port 303 coupler 304 port 305 and fibre 307. A portion
of illuminating light emerging from exit end 308 is substantially coherent
and substantially uncollimated and is directed by lens 309 into beam
splitter 310. About 90% of the illuminating light entering beam splitter
310 passes directly through it to lens 311 where it is focussed into a
spot 312 which intersects sample 313. Outgoing light resulting from the
interaction between the focussed spot 312 and object 313 is collimated by
lens 311 and enters beam splitter 310. The intensity of the outgoing light
corresponds to the strength of the interaction between the spot 312 and
object 313 at the particular spot position and can be used to characterise
a particular parameter of the object at that particular position such as
reflectance. Most of the outgoing light passes through beam splitter 310
to be focussed by lens 309 into the core of fibre 307 at fibre end 308. A
substantial portion of this outgoing light passes to detector 316 via
fibre 307, port 305 coupler 304 port 314 and fibre 315. Detector 316
detects the outgoing light and sends a signal corresponding to the
intensity of outgoing light to the imager 330 via line 331. About 10% of
the illuminating light entering beam splitter 310 is directed into the
core of single mode fibre 318 by lens 317 and thence into coupler 320 via
port 319. A portion of the coherent illuminating light from laser diode
301 passes out port 306 of coupler 304 via fibre 302 and port 303. This
reference light passes along single mode fibre 327 and into coupler 320
via port 328 where it interferes with illuminating light present in
coupler 320. The interference light intensity passes to detectors 325 and
326 via fibres 321 and 322 and ports 323 and 324. Detectors 325 and 326
detect interference light intensity and each output a signal to calculator
329 corresponding to the relative phase difference between the reference
light and the illuminating light. If the z position of fibre end 308
changes the interference light intensity at detectors 325 and 326 also
changes as a result of the change in the phase difference between the
reference light and the illuminating light. This change in the phase
difference is as a result of the path length change between fibre end 308
and the entrance end of fibre 318. Calculator 329 determines the z
position change of fibre end 308 and hence the z position of spot 312 from
the change in the interference light intensities at detectors 325 and 326.
Microscope 300 can be utilised to obtain a three dimensional image of
object 313 by moving fibre end 308 to and fro along the z axis using
piezoelectric stack 333 and moving object 313 in the x and y directions.
The three dimensional image is captured and stored as follows. The signal
at detector 316 is passed along line 331 to imager 330 which stores the
value for that x y z coordinate. Piezoelectric stack 333 then moves fibre
end 308 such that the phase difference measured by calculator 329 changes
by a predetermined amount corresponding to a known change in the z
position of fibre end 308 and a known change in the z position of spot
312. This new spot position is noted by imager 330 and the signal on
detector 316 which is passed along line 331 is stored for the new x y z
coordinate. This procedure is repeated for all required z positions at a
given x y coordinate on object 313. Mechanical stage 334 is utilised to
move object 313 so that spot 312 can be scanned in the z direction at
different x y coordinates on object 313 until a three dimensional image of
object 313 has been stored by imager 330.
Alternatively microscope 300 can be operated in the following manner. Fibre
end 308 is oscillated sinusoidally and rapidly in order to z scan spot 312
through the surface of object 313. Thus the signals at detectors 325 and
326 take the form of sine waves with sinusoidally varying frequencies
(i.e. frequency modulated sine waves). Each peak on the sine waves
corresponds to a single wavelength of translation of fibre end 308. A
simple way to keep track of the z position of fibre end 308 to a
resolution of one wave length of the light being used is to count the
peaks which correspond to the interference fringes.
Note that depending on the relationship between the characteristics of
optical elements 309 and 311, one wavelength of movement of fibre end 308
may correspond to 1/100 of a wavelength of movement of diffraction limited
spot 312. Thus by monitoring the intensity level at detector 316 and
noting the interference signal at the time of the peak signal at detector
316 in imager 330 one can determine the position of the surface of object
313 to 1/100th of a wavelength or better. In this case it is the signal at
detector 316 rather than the position of fibre end 308 that determines
resolution.
The detection system of FIG. 3 incorporating coupler 320 fibres 321 and 322
and detectors 325 and 326 can be replaced with detection system 400 shown
in FIG. 6. Note that fibres 318 and 327 in FIG. 3 correspond to single
mode fibres 401 and 402 in FIG. 6. Illuminating light from fibre 401
emerges from fibre end 403. Reference light from fibre 402 emerges from
fibre end 404. Fibres 401 and 402 are substantially parallel in the
vicinity of their ends. Fibre ends 403 and 404 are in close proximity to
one another so that illuminating light emerging from end 403 interferes
with reference light emerging from end 404 to form fringe pattern 405. As
the phase of the light emerging from fibre end 403 changes with respect to
that emerging from fibre end 404 fringe pattern 405 translates across dual
element detector 406. If the phase change of the light emerging from fibre
end 403 relative to the light emerging from fibre end 404 is positive,
fringe pattern 405 moves in one direction, up say. If the phase change of
the light emerging from fibre end 403 relative to the light emerging from
fibre end 404 is negative fringe pattern 405 moves in the opposite
direction, down say. If the fringe pattern is moving up the bottom half of
dual detector 406 detects a particular feature in the signal before the
top half. On the other hand, if the fringe pattern is moving down the top
half of dual detector 406 detects a particular feature in the signal
before the bottom half. In this manner, referring back to FIG. 3, the
direction and magnitude of the z movement of fibre end 308 can be
determined.
A moving fibre system 600 illustrated in FIG. 4 has laser 601, and coupler
603 which is linked to laser 601 by single mode optical fibre 602.
Coherent illuminating light enters fibre 604 from coupler 603. Fibre 604
is attached near its exit end 605 to x y z vibrator 606. Light emerging
from fibre end 605 is coherently directed in an uncollimated manner
preferentially in directions x y and z. This can be done, for example, by
careful geometric shaping of fibre end 605. Light directed in the z
direction is collected by lens 607 and focussed into end 608 of single
mode fibre 609. Light directed in the y direction is collected by lens 610
and focussed into end 611 of single mode fibre 612. Light directed in the
x direction is collected by lens 613 and focussed into end 614 of single
mode fibre 615. Z reference light from laser 601 is directed into single
mode fibre 616 by coupler 603. Fibre 616 has along its length a
piezoelectric cylinder 617 connected to detector/calculator 619 by line
618. Fibre 616 is connected to coupler 620 which in turn is coupled to
detector/calculator 619 by fibre 621. Y reference light from laser 601 is
directed into single mode fibre 622 by coupler 603. Fibre 622 has along
its length a piezoelectric cylinder 623 connected to detector/calculator
619 by line 624. Fibre 622 is connected to coupler 625 which in turn is
coupled to detector/calculator 619 by fibre 626. X reference light from
laser 601 is directed into single mode fibre 627 by coupler 603. Fibre 627
has along its length a piezoelectric cylinder 628 connected to
detector/calculator 619 by line 629. Fibre 627 is connected to coupler 630
which in turn is coupled to detector/calculator 619 by fibre 631. All of
the fibres and couplers in device 600 are single mode coherence
maintaining.
In operation coherent light from laser 601 is coupled into single mode
fibre 602. Coherent light from fibre 602 enters 5 port coupler 603. A
portion of this light is coherently injected into fibre 604 by coupler
603. A portion of the light emerging from fibre end 605 of fibre 604 is
directed coherently in the z direction towards lens 607. Lens 607 then
focusses a portion of this light into the core of fibre end 608 of single
mode fibre 609. This light travels coherently along fibre 609 and enters
coupler 620. Another portion of the light entering coupler 603 is injected
coherently into reference fibre 616. Light travels along fibre 616 to
coupler 620 where it interferes with the illuminating light from fibre
609. The light intensity resulting from the interference of light from
fibres 609 and 616 is injected into fibre 621 and detected by
detector/calculator 619. Detector/calculator 619 generates an error signal
which is fed via line 618 to piezoelectric cylinder 617 to maintain the
interferometer in quadrature by causing the piezoelectric cylinder to
change diameter and thus physically alter the length of reference fibre
616. The above procedure allows one to monitor the change in the z
position of fibre end 605.
The position of fibre end 605 in the x and y directions is determined in a
similar manner to that described for the z direction.
X y z vibrator 606 moves fibre end 605 in the x y and/or z directions.
Detector/calculator 619 determines the change in x y and/or z position of
fibre end 605 as described above and stores the result. Following the
above procedure system 600 can be used to scan an object in a
dimensionally known manner.
Refractometer 500 is illustrated in FIG. 5. Laser 501 is linked to coupler
502 with single mode fibre 503. Illuminating light enters fibre 504 from
coupler 502 and emerges from fibre end 505 located in refractometer sample
cell 506. Focussing reflector 507 is disposed in cell 506 to direct light
from fibre end 505 back into the core of fibre 504 via end 505. A portion
of the illuminating light entering coupler 502 is directed into fibre 508
by coupler 502. Fibre 508 is wrapped around piezoelectric cylinder 509
between coupler 502 and coupler 512. Piezoelectric cylinder 509 is
connected to detector/calculator 510 by line 511. Illuminating light back
reflected from mirror 507 passes through fibre 504 to coupler 502 which
directs a portion into fibre 513 which is also connected to coupler 512.
Coupler 512 is linked to detector/calculator 510 by optical fibres 514 and
515. Cell 506 has inlet port 516 and outlet port 517. While all of the
fibres and couplers in refractometer 500 are designed for single mode
operation they could equally well be multi mode fibres and couplers.
In operation coherent light from helium neon laser 501 is injected into
fibre 503, directed into coupler 502 and split between illuminating fibre
504 and reference fibre 508. Illuminating light leaves the core of fibre
504 at fibre end 505. Focussing reflector 507 reflects and focusses a
significant portion of the coherent uncollimated illuminating light back
into the core of fibre 504 at fibre end 505. The light path between fibre
end 505 and reflector 507 is contained entirely within cell 506 and the
light path length depends on refractive index of the substance in cell 506
(which can be a gas or liquid for example). The back reflected light
focussed into the core of fibre 504 passes into coupler 502 which directs
a portion into fibre 513. The light entering fibre 513 is directed into
coupler 512. Light entering reference fibre 508 passes around
piezoelectric cylinder 509 into coupler 512 where it interferes with the
back reflected light from fibre 513 to produce a light intensity
characteristic of the phase difference between the back reflected and
reference beams. The interference light intensity is injected into fibres
514 and 515 by coupler 512 and passes to detector/calculator 510.
Detector/calculator 510 generates an error signal which is fed via line
511 to piezoelectric cylinder 509 to maintain the phase of the reference
beam relative to the back reflected beam in quadrature by causing the
piezoelectric cylinder 509 to change diameter and thus physically alter
the length of fibre 508. Detector/calculator 510 determines the refractive
index change of the material passing through the cell 506 from the error
signal.
As indicated the above, the energy source utilised in the method and
apparatus of the invention may be coherent or partially coherent. In a
white light interferometer analyser 700 illustrated in FIG. 7, light of
wavelength L1 from super luminescent diode 701 is injected into integral
single mode optical fibre pigtail 702 which is connected to white light
interferometer cell 703 having a path length difference between the signal
and reference beams whose magnitude D depends on the strength of the
parameter being measured. The output of cell 703, comprising the
recombined signal and reference beams from cell 703, is injected to single
mode fibre 704 which is connected via port 706 to single mode wavelength
division multiplexer 705. With respect to port 706 wavelength division
multiplexer 705 has wavelength L2 tap on port 709 and L1 and L2 output
port 707. Port 708 is antireflection terminated. Long coherence length
laser diode 710, emitting light with wavelength L2, is connected to port
709 by single mode fibre 711. Port 707 is connected to single mode
wavelength independent coupler 712 via single mode fibre 713 and port 714.
Coupler 712 has output ports 715 and 716 and antireflection terminated
port 717. Note that multiplexer 705 could be dispensed with by injecting
light from diode 710 directly into port 717. Port 716 is connected to
single mode fibre 718 having exit end 719. A portion of short coherence
length light of wavelength L1 and long coherence light of wavelength L2
emerges from end 719 with a numerical aperture of typically 0.1 to be
collected by lens 720 and injected with high numerical aperture into
entrance end 721 of single mode fibre 722 which is connected to port 724
of wavelength independent single mode fibre coupler 723 which has other
entrance port 725, exit port 727 and antireflection terminated port 726.
Port 715 is connected to port 725 by single mode fibre 729. Port 727 is
joined to wavelength division multiplexer 730 via single mode fibre 728
and entrance port 731. With respect to entrance port 731, multiplexer 730
has wavelength L1 exit port 732, wavelength L2 exit port 734 and
antireflection terminated port 733. Port 732 is connected to avalanche
photodiode 738 by fibre 737. Port 734 is connected to pin diode 736 by
fibre 735. End 719 can be scanned towards and away from lens 720 by
scanner 739 and/or end 721 can be scanned towards and away from lens 720
by scanner 739a such that the path length difference taken by light
passing from coupler 712 to coupler 723 via lens 720 and that passing via
fibre 729 can at least be varied between -D and +D. Diode 736, diode 738
and scanner 739 and/or are connected to computer 740 by lines 751, 752 and
753 respectively. Computer 740 is connected to recorder 741 by line 742.
In operation, partially coherent light of average wavelength L1 from diode
701 is injected into cell 703 via fibre 702 where it is split into two
beams which travel different paths with a length difference of D before
being injected into fibre 704. A first portion of this L1 wavelength light
is guided to end 719 via port 706, multiplexer 705, port 707, fibre 713,
port 714, coupler 712, port 716 and fibre 718 from which it emerges
uncollimated and piecewise coherent with a path length difference of D
between the pieces and is collected with a low numerical aperture and
directed with a high numerical aperture by lens 720 into end 721 of fibre
722 which guides it to coupler 723 via port 724. A second portion of the
light of wavelength L1 injected into fibre 704 by cell 703 is guided to
coupler 723 via port 706, multiplexer 705, port 707, fibre 713, port 714,
coupler 712, port 715, fibre 729 and port 725 where it interferes with the
first portion to produce temporal fringes of variable visibility which
depends on the relative position of ends 719 and 721. The result of this
interference is directed to diode 738 via port 727, fibre 728, port 731,
multiplexer 730, port 732 and fibre 737. The intensity of the signal
produced by diode 738 is monitored by computer 740. The relative position
of fibre ends 719 and 721 is determined by computer 740 by monitoring the
fringes produced by interference in coupler 723 between light of
wavelength L2 travelling along a first path from fibre 711 to coupler 723
via port 709, multiplexer 705, port 707, fibre 713, port 714, coupler 712,
port 716, fibre 718, end 719, lens 720, end 721, fibre 722 and port 724
and a second path from fibre 711 to coupler 723 via port 709, multiplexer
705, port 707, fibre 713, port 714, coupler 712, port 715, fibre 729 and
port 725. The interference signal produced in coupler 723 by light of
wavelength L2 is directed to diode 736 via port 727, fibre 728, port 731,
multiplexer 730, port 734 and fibre 735 where it is detected. To determine
D, and thus the magnitude of the parameter being measured, computer 740
directs scanner 739 and/or 739a via line 743 to move fibre end 719 and/or
721 axially with respect to stationary lens 720 while monitoring the
position of fibre end 719 and/or 721, via the signal on diode 736 via line
751, and noting the signal on diode 738 via line 752. The computer then
correlates the signal from diode 738 as a function of the position of
fibre end 719 and/or 721, for example through the use of Fourier
transforms to obtain the path imbalance D, and thus the magnitude of the
parameter being measured, very accurately. The result is recorded in
recorder 741 via line 742.
INDUSTRIAL APPLICABILITY
Apparatuses and methods for measuring a change in an energy path Length
according to invention facilitate the measurement of the apparent position
of the end of a fibre in one, two or three dimensions, through which
illuminating light emerges, relative to another object.
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